Detection of infrared light by normal incidence detection typically uses semiconductor p-n junctions or conductors in which light generates carriers within the bulk of the semiconductor that is irradiated. In such detectors, carriers (electrons and holes) are generated if an incident photon has a higher energy than the band-gap, and is absorbed within the semiconductor to contribute to an overall detector current. This approach limits the use of detectors to energies above their bandgap, and, in the case of silicon, this limits an efficient detection of light to above approximately 1.1 eV.
As a result of this limitation, silicon PIN and photodetectors are typically used in visible and NIR wavelength ranges. For longer wavelengths, other materials such as InGaAs or Ge have to be employed, and this typically results in complex materials systems that have to be grown or waferbonded together resulting in infrared detector arrays that are not cost effective